In a typical example of resistance spot welding, a pair of electrodes clamps two or more pieces of materials together by a predetermined force, and passes weld current between the tips of the electrodes through the pieces of materials. As the weld current flows through the pieces of materials, the resistance of the materials to the current flow causes the materials to heat to their inherent melting point. The resulting molten material solidifies under the predetermined clamping force to form the welded joint, or nugget.
Conventional resistance spot welding processes used to weld two or more pieces of sheet materials together may apply alternating current (AC) or direct current (DC). The operational current range is defined as the weld current values between the weld current for the designed minimum weld size (the minimum weld current) and the expulsion weld current (the maximum weld current). The weld current input may be one or more pulses. The time of each weld current pulse may range from one cycle per second to sixty cycles or more per second.
The weld current range is defined as the difference between the lower limit (i.e. the minimum) weld current required to produce the minimum weld nugget size and the upper limit (i.e. the maximum) weld current which causes molten metal splashing. Resistance spot welding (RSW) weldability tests have revealed that when DC weld current mode is employed there is no stable weld current range for thin gauge (0.91 mm) USIBOR® 1500P and a very narrow weld current range for 1.52 mm USIBOR® 1500P. RSW weldability tests have also shown that when AC weld current is used there is a stable weld current range. Experimental results indicate that the deterioration rate of the electrode tip face for DC is much higher than that for AC. The use of higher weld force, longer weld time and larger size electrodes may enlarge the weld current range for DC welding. However, the experimental results also discovered that the improvement for electrode life is very limited from welding parameter optimization.
Both low frequency direct current (DC) resistance welding equipment and middle frequency direct current (MFDC) resistance welding equipment generate constant secondary DC current output for welding. The middle frequency direct current (MFDC) resistance welding equipment utilizes frequency pulses of 400 to 2,500 Hz instead of the frequency of base alternating current (50 or 60 Hz) to transform primary current into secondary current. Thus, the size of MFDC welding equipment is significantly reduced compared to AC and low frequency DC welding equipment. The output welding current of MFDC resistance welding equipment remains constant. Moreover, the MFDC welding equipment does not cause power supply line disturbances as is the case with low frequency DC and AC welding equipment.
MFDC resistance spot welding equipment is widely used in automotive, appliance and aircraft manufacturing industries because of its small size, light weight and controllability, and it is particularly suitable for robotic applications. On the other hand, the size, weight, and/or control of AC RSW equipment is not suitable for the same applications. Therefore, it would be advantageous to develop an innovative resistance spot welding method to obtain a robust resistance spot welding process with enlarged weld current ranges, extended electrode life, fine microstructure in the weld nugget, excellent welded joint strength, or any combination of these features.